A turbine bypass valve and a temperature spray valve are control valves utilized in industrial plants, and the controlling technology thereof is generally performed through a PID controller. The PID controller has a set value (SV value) and a process value (PV value) entered therein, and calculates a control instruction value (MV value) through a feedback control in such a way that the PV value becomes equal to the SV value.
Some control valves among the control valves utilized in industrial plants are designed to avoid a case in which the MV value becomes 100%, i.e., the control value is fully opened. Such control valves are utilized as turbine bypass control valves in a combined cycle power generating plant that is a combination of a gas turbine, a steam turbine, and an exhaust heat recovery boiler.
The turbine bypass control valve is provided with a PID controller controlling the same, and an SV value and a PV value are entered in this PID controller. The PID controller calculates an MV value so as to make the PV value approach the SV value to be consistent with the SV value. The MV value is an open level instruction for the turbine bypass control valve, and this valve is opened in accordance with this instruction. According to such PID controllers, when the SV value is set high, the MV value decreases, and when the SV value is set low, the MV value increases.